1. Field of the Invention
The present invention pertains to Fresnel lenses for use as a component of a viewing screen for projection systems, specifically as an element of a screen for a multi-tube rear projection system. The Fresnel lens has facets with riser angles which vary as a function of their radial distance from the center of the lens, the variation being approximated by a linear function. The Fresnel lens according to the present invention has an increased efficiency over lenses according to the prior art, providing high resolution capability.
2. Description of the Prior Art
Consumer based rear projection television (PTV) systems typically include three CRTs which form the red, green and blue portions of the TV picture. Three projection lenses magnify these images and converge them into a single plane. The viewing screen is placed in this plane.
Most rear PTV systems utilize a two part screen in which a lenticular array and black stripes are incorporated into the front part while a Fresnel lens is arranged on the rear part of the screen. The linear lenticular array distributes light horizontally and black stripes may be introduced to increase the image contrast by reducing the ambient room light reflections. The registration between the black strips and the lenticular array must be controlled with high accuracy so that the TV picture is unaffected by the presence of the stripes. The Fresnel lens directs the projected light toward the viewing audience through a diffuser which is incorporated into the screen for vertical spreading. The amount of horizontal and vertical spreading defines the dimensions of the audience space. Screens in the prior art have been made in either one or two pieces, and for rear or front projection.
The optical performance of the conventional black stripe screens is limited in terms of the size of the audience space, color shift, resolution, mechanical stability and other factors.
The purpose of the Fresnel lens is to increase the field of view of the PTV system. In effect it acts as a field lens. It is designed to image the exit pupil of the projection lens to the plane of the viewing audience. Without a field lens, viewers in the center of the viewing space would only see the information from the center of the cathode ray tube. With the field lens, the rays are bent toward the viewer so that he can see the entire picture. Since a Fresnel lens is located at the image plane of the projection lenses, it introduces no aberrations.
Further information about PTV systems and in particular rear projection TV screens may be had from U.S. patent application Ser. No. 567,101, filed Dec. 30, 1983, now U.S. Pat. No. 4,573,764 which is hereby incorporated by reference.
Projection TV systems and Fresnel lenses for use therein have been known in the prior art.
Typically, the Fresnel lenses used in the display apparatus whose riser angles are constant. In U.S. Pat. No. 4,391,495 it is mentioned in passing that the riser angle "is coincident with a pencil of light traveling through the system to the exit pupil". There is no appreciation in U.S. Pat. No. 4,391,495 that this ideal solution cannot be achieved with an extended light source.